Until now, a condensation-curable polyorganosiloxane has been widely used for an adhesive, water-proof and moisture-proof coating material, electrical insulator film, encapsulating material for architecture, etc. In recent years, use for an encapsulating material of a light emitting diode (LED) has attracted attention from the viewpoint of high heat resistance, light resistance, and transparency thereof. A condensation-curable organopolysiloxane, however, has lower reactivity and thus poor productivity compared with an addition-curable polyorganosiloxane. There is a problem that the silicone resin cannot exhibit an inherent high heat resistance and light resistance since a large amount of condensation catalyst for enhancing the reactivity accelerates the deterioration of the silicone resin. In addition, some catalysts are unsuitable for a field requiring transparency since they have a color themselves or exhibit a color due to the deterioration.
So far, many attempts have been made to improve and utilize a condensation-curable polyorganosiloxane. For example, Patent Literature 1 attempts to minimize the deterioration of a resin while enhancing the curing rate by adding, besides a metal catalyst such as aluminum or zinc, a condensation catalyst such as a phosphate ester or a boron compound to a polyorganosiloxane having two or more silanol groups per molecule and a polyorganosiloxane having two or more silicon-bonded alkoxy groups per molecule. In addition, as described in Patent Literature 2, there has been an attempt to decrease catalyst remained in a cured product by adding a volatile amine catalyst to cure partially hydrolyzed tetraalkoxysilane or trialkoxysilane and linear polyorganosiloxane containing silanol groups at the both terminals. Further, as described in Patent Literature 3, there also has been an attempt previously increasing the molecular weight of a condensable polyorganosiloxane to reduce the number of reaction for gelation.